This invention relates to an image processing apparatus and method and, more particularly, to an image processing apparatus and method for performing image formation using a plurality of image forming units.
In a conventional image forming apparatus capable of forming a full-color image, a plurality of color toner images are formed on an image carrier in an image forming unit, and are sequentially transferred onto a recording medium to overlap each other, thereby forming a full-color image. Therefore, in order to stabilize the image quality of an output image, the toner images to be transferred to overlap each other must always maintain a stable density balance.
As a method of stabilizing the density balance of an output image, the following methods are known.
For example, upon completion of the warm-up operation immediately after the image forming apparatus is started, specific patterns in units of colors are formed on an image carrier such as a photosensitive body or photoconductor, and light emitted by a predetermined LED and reflected by each of the specific patterns in units of colors is measured by a sensor such as a photodiode, thus detecting the toner densities of the respective colors. When it is determined based on the measurement result that the balance of the toner densities is not stable, the measurement result is fed back as an image forming condition, e.g., .gamma. correction, thereby improving stability of the final output image quality.
Furthermore, in another method, even when the image forming characteristics change due to various environmental variation factors, a specific pattern is formed on the image carrier in correspondence with the environmental variation amount, and the density of the specific pattern is read in the same manner as the above-mentioned method. The measurement result is fed back as an image forming condition, e.g., .gamma. correction, thus improving stability of the final output image quality as well.
However, conventionally, an image forming apparatus having a plurality of image carriers for respectively forming a plurality of color toner images does not give sufficient consideration to precise detection of the states of the image carriers and feeding back of the detection results for correction control of image data.
Also, controlling the image forming condition of such image forming apparatus by effectively using an image reading means for supplying image data is also not given sufficient consideration.
On the other hand, if the method of reading the density of a specific pattern on an image carrier is applied to an image forming apparatus having a plurality of image carriers for respectively forming a plurality of color toner images, the characteristics of means (to be referred to as toner image reading means hereinafter) for reading light reflected by toner images of specific patterns on the respective image carriers must be substantially absolutely equivalent to each other with respect to the respective colors.
Note that the "absolutely equivalent" state is a state wherein the toner image reading means for respectively reading light reflected by, e.g., yellow, magenta, and cyan toner images used in image formation detect equal density values when they read a gray scale in an achromatic color formed by evenly superposing the three colors.
When the toner image reading means are not absolutely equivalent to each other with respect to the respective colors, an image having a poor color balance is output.
It is possible to some extent to adjust the toner image reading means to be absolutely equivalent to each other for the respective colors upon assembling of the image forming apparatus. However, when the toner image reading means for a certain color is exchanged due to, e.g., a failure, it is very difficult to adjust the characteristics of the exchanged toner image reading means to be absolutely equivalent to those of the other toner image reading means.
As for the respective image forming units, the density obtained by reading a specific pattern on each image carrier (photosensitive drum) does not always match the density of an image actually output onto a recording medium (paper) after the image forming units have been used over a long period of time.
For example, when a cleaning blade has been in sliding contact with the image carrier over a long period of time so as to clean the residual toner on the image carrier upon transfer, the surface of the image carrier becomes rough and the relationship between the adhesion amount of toner and the reflected light amount changes from that in an initial state. Such change also takes place when the optical characteristics of the toner image reading means change due to, e.g., adhesion of toner, dust, and the like to an optical window for protecting an optical element.
Therefore, an image forming apparatus having a plurality of image forming units must have a means for strictly correcting the characteristics of the toner image reading means themselves, and their relationship. When the image forming apparatus does not have any correction means, an output image with an optimal color balance cannot be obtained when the toner image reading means is exchanged or when the apparatus is used over a long period of time.
The above-mentioned problems will be described in detail below while taking as an example a full-color image forming apparatus having one image forming unit.
As the image forming apparatus having one image forming unit, an image forming apparatus which has one image carrier and a transfer drum, and sequentially transfers and outputs respective color toner images formed in turn on the image carrier onto a recording medium carried on the transfer drum will be examined. Such image forming apparatus has only one toner image reading means, and hence, the respective colors are read by a single sensor within a predetermined photosensitivity range. For this reason, it is checked whether a certain relative density ratio is obtained among the densities of the colors even when, for example, the sensitivity of this sensor deviates from average sensitivity.
A case will be examined below wherein the following density correction control operations are performed based on the toner densities of colors read by a single toner image reading means.
(1) The maximum densities are determined in units of colors of toner images formed on the image carrier.
(2) The linearity of the toner image density with respect to the laser emission time (or emission amount) of a toner image formed on the image carrier is maintained in units of colors.
(3) The toner adhension amounts (fogging amounts) are controlled in units of colors of toner images formed on the image carrier.
Of the above-mentioned three control operations, since the ratio of the density to the emission time in (2) can be attained even by a single toner image reading means, for example, an achromatic gray scale can be easily formed.
However, as for determination of the maximum toner density values in (1) and fogging amount control in (3), since the respective colors cannot be evaluated based on their absolute amounts, the maximum density in each color toner image cannot be determined, and the fogging amount cannot be appropriately corrected.
On the other hand, in the image forming apparatus having a plurality of image forming units corresponding to colors, the constituting elements such as photodiodes that constitute toner image reading means in the respective image forming units have a difference although the difference falls within a tolerance. For this reason, the image forming units of the respective colors have different maximum toner density values in (1), and hence, it is difficult to form an achromatic gray scale. Also, the fogging amount control in (3) is not sufficient, and for example, chromatic fog may be generated upon formation of a gray scale image.